Downhole deposition monitoring system

ABSTRACT

Described is a downhole apparatus for detecting and removing deposits from a surface exposed to wellbore fluids. The apparatus can monitor the rate of deposition and subsequently remove the deposited material. The combination of detection apparatus and removal apparatus provides a downhole instrument with self-cleaning operation mode.

This invention relates to apparatus and methods for monitoring soliddeposits of material in a wellbore and operating downhole sensors andother wellbore equipment. Particularly, the invention relates to suchapparatus and methods for sensing and removing solid deposits inhydrocarbon wells.

BACKGROUND OF THE INVENTION

The formation of both organic and inorganic deposits in the nearwellbore region of producing formations and on the tubing of a producinghydrocarbon well can be a major and costly problem. See, e.g. Allen, T.O. and Roberts, A. P., Production Operations, Vol. 2, 2nd edition, pp.11-19 and 171-181, OGCI, Tulsa, Ok. (1982); Cowan, J. C and Weintritt,D. J., Water-formed Scale Deposits, Gulf Publishing Co., Houston (1976).The deposits can seriously impede the productivity of wells by reducingthe near wellbore permeability of producing formations and progressivelyrestrict the diameter of the tubing.

The formation of inorganic deposits, or scale, is caused by theprecipitation of inorganic salts from produced water. Calcium carbonatescale is usually formed by the change in the pressure and temperature ofthe produced water in the near wellbore and in the production tubing.Barium, strontium and calcium sulphate scales are usually formed by themixing of formation water and seawater injected into producing wells;the high concentration of sulphate in seawater mixes with the highconcentrations of divalent cations in formation waters with theresulting precipitation of the sulphate salts. The formation of scalemay be partly prevented by water shut-off treatments and the use ofscale inhibitors. Once formed, scale can be removed only with somedifficulty; calcium carbonate scale can be dissolved by mineral acidsand barite scale can be removed by milling or scale dissolvers such asEDTA. See, e.g. Putnis, A, Putnis, C. V. and Paul, J. M., “Theefficiency of a DTPA-based solvent in the dissolution of barium sulfatescale deposits”, SPE International Symp. Oilfield Chemistry, SanAntonio, Tex., February 1995, SPE 29094. In extreme cases the productiontubing must be removed and replaced, although the presence ofradioactive scale (due to the presence of radium salts) can make scaledisposal an environmental issue.

The production of hydrocarbons frequently causes the precipitation oforganic precipitates such as paraffin waxes and asphaltenes. Theseorganic precipitates are caused by changes in the pressure andtemperature of the produced fluids in the near wellbore. Theprecipitates can be removed with solvent washes, although the disposalof the solvent after cleaning represents an increasing environmentalproblem.

The solubility of various inorganic and organic species can be predictedfrom thermodynamic models of the electrolyte solutions or hydrocarbons.See, e.g. Jasinski, R, Taylor, K. and Fletcher, P., “Calcitescaling—North Sea HTHP wells”, SPE Symp. Oilfield Scale, Aberdeen,January 1999; Calange, S., Ruffier-Meray, V. and Behar, E., “Onsetcrystallization temperature and deposit amount for waxy crudes:experimental determination and thermodynamic modelling”, SPEInternational Symp. Oilfield Chemistry, Houston, Tex., February 1997,SPE 37239. However, thermodynamic models are essentially equilibriummodels and they cannot predict any details of the precipitation processsuch as the location of precipitation, the rate of precipitation or thedegree of supersaturation that the fluid can tolerate.

Several patents and papers have described both acoustic and non-acousticmethods for sensing the formation of scale in producing hydrocarbonwells and similar environments. An acoustic method for measuring thethickness of metal oxide corrosion products on the inside of boilertubes has been described. See Lester, S. R., “High frequency ultrasonictechnique for measuring oxide scale on the inner surface of boilertubes”, U.S. Pat. No. 4,669,310, Jun. 2, 1987. The thickness of oxidescale is determined by the time of flight of an acoustic pulse appliedfrom the external surface of the pipe. The frequency of the acousticpulse was 50 MHz, which enabled a scale thickness of approximately 0.1mm to be detected. The use of an automated ultrasonic inspection systemfor determining the thickness of scale formation which has formed on theinside of heat transfer tubes in boilers has been described. See Okabe,Y., Iwamoto, K., Torichigai, M., Kaneko, S., Ichinari, J. and Koizumi,K., “Automated ultrasonic examination system for heat transfer tubes ina boiler”, U.S. Pat. No. 4,872,347, Oct. 10, 1989. A rotating transducerwas inserted into the tubes and their diameter as a function of locationdetermined by the reflection of sound from the scale-water interface. Anacoustic wireline logging tool, Schlumberger's Cement Evaluation Tool(CET), has been used to determine the accumulation of scale on thecasing of geothermal wells. See, U.S. Pat. No. 5,072,388, to O'Sullivanet al. The interfaces between the scale and wellbore fluid and the scaleand the casing are determined by the transit time of the acoustic waves,which have a frequency of approximately 0.5 MHz. U.S. Pat. No. 5,092,176disclosed a method for determining the thickness of scale on the insideof a water pipe by the attenuation of acoustic energy emitted andreceived by a transducer on the outside of the pipe. The optimumfrequency range for the ultrasound was observed to be 3-7 MHz. Forexample, measurements made using ultrasound below a frequency of 3 MHzgave poor sensitivity to scale thickness and beam spreading was observedto be a problem. An acoustic method of identifying scale types and scalethickness in oil pipelines using the attenuation in the reflectedacoustic signal from a tool that is moved through the inside of the pipehas been described. See, Gunarathne, G. P. P. and Keatch, R. W., “Noveltechniques for monitoring and enhancing dissolution of mineral depositsin petroleum pipelines”, SPE Offshore Europe Conference, Aberdeen,September 1995, SPE 30418 (hereinafter “Gunarathne”); Gunarathne, G. P.P. and Keatch, R. W., “Novel techniques for monitoring and enhancingdissolution of mineral deposits in petroleum pipelines”, Ultrasonics,34, 411-419 (1996) (hereinafter “Gunarathne and Keatch”). The frequencyof the ultrasound used was in the range 3.5-5.0 MHz, which allowed thethickness of barium sulphate scale on steel to be measured to anaccuracy of ±0.5 mm.

U.S. Pat. No. 5,661,233 (hereinafter “Spates et al.”) disclosed severalacoustic-wave devices for determining the deposition of organicprecipitates, such as paraffin wax, on to a sensing surface immersed ina petroleum-based fluid. The acoustic measurements were made withdevices using various acoustic modes: surface acoustic waves andthickness shear, acoustic plate and flexural plate modes. The devicesmeasured changes in the damping voltage and resonant frequency of thedevice as the wax precipitate formed, although no details were disclosedregarding the operating frequencies of the acoustic devices or theacoustic power they generated. Spates et al. discussed periodic cleaningof the sensing surface of the acoustic device by heating the surface tomelt the paraffin wax. However, the use of acoustic energy to clean theorganic precipitates from the acoustic sensor was not disclosed orsuggested. Several applications of the measurement of wax accumulationwere described, including location of acoustic devices on the sea floorto monitor the production of hydrocarbon from oil wells and guide welltreatments. The application of a quartz microbalance to measuresimultaneously mass loading and liquid properties has been described.See, U.S. Pat. No. 5,201,215; Martin, S. J., Granstaff, V. E. and Frye,G. C., “Characterisation of a quartz crystal microbalance withsimultaneous mass and liquid loadings”, Anal. Chem., 63, 2272-2281(1991) (collectively hereinafter “Granstaff and Martin”). The authorsused changes in the resonant frequency and magnitude of the maximumadmittance of a quartz microbalance to differentiate between changes inthe mass of material deposited from a liquid and changes in theproperties of the liquid (density and viscosity). The operatingfrequency of the quartz microbalance was close to 5 MHz, at which valuethe resonator was able to detect solid films of the order of 0.1 μm inthickness.

The application of an on-line quartz crystal microbalance to monitor andcontrol the formation of organic and inorganic precipitates fromhydrocarbons and water has been-described. See, U.S. Pat. No. 5,734,098(hereinafter “Kraus et al.”). The quartz crystal microbalance consistedof a thickness-shear mode resonator and was described by Kraus et al. asbeing substantially similar to those disclosed by Granstaff and Martin.Kraus et al. described the use of the thickness-shear mode resonator forthe on-line measurement of scaling, corrosion and biofouling inindustrial processes. They also described the use of the measurement ofdeposit formation to determine the treatment required to correct theindustrial process and prevent continual deposit formation, e.g., by useof a chemical additive such as an inorganic scale inhibitor. AlthoughKraus et al. described the use of a thickness-shear mode resonator tomonitor deposit formation from hydrocarbons, industrial water and theirmixtures (including emulsions), no disclosure or suggestion was made tothe operation of these sensors in or near producing hydrocarbon wells,on either a temporary or permanent basis. Additionally, there was nodisclosure or suggestion regarding the treatment of producinghydrocarbon wells, for either organic or inorganic deposit formation, onthe basis of measurements made by these sensors.

A laboratory acoustic resonance technique to determine the onset of theprecipitation of wax and asphaltene from produced hydrocarbons has beendescribed. See, Sivaraman, A., Hu, Y., Thomas, F. B., Bennion, D. B. andJamaluddin, A. K. M., “Acoustic resonance: an emerging technology toidentify wax and asphaltene precipitation onset in reservoir fluids”,48th Annual Tech. Meet. The Petroleum Society, Calgary, Canada, Jun.8-11, 1997, paper CIM 97-96; Jamaluddin, A. K. M., Sivaraman, A., Imer,D. Thomas, F. B. and Bennion, D. B., “A proactive approach to addresssolids (wax and asphaltene) precipitation during hydrocarbonproduction”, 8th Abu Dhabi Intern. Petroleum Exhibit., Abu Dhabi, U. A.E., 11-14 Oct. 1998, SPE 49465. The spectrum of resonant frequencies ofa sample of liquid hydrocarbon in a cylinder of fixed length andcross-sectional area was obtained as a function of temperature andpressure. The resonant spectra were collected using two ultrasonictransducers operating over the frequency range 0-40 kHz, althoughsignificant resonances were observed only over the frequency range 5-35kHz. The precipitation of wax and/or asphaltene from the liquidhydrocarbon sample as the pressure and temperature of the sample waschanges resulted in changes in the frequency of the resonances andchanges in their amplitude. The changes in the resonant spectra wereattributed to changes velocity of sound in the liquid. No reference wasmade to the precipitation of wax or asphaltene on the transmitting orreceiving transducer.

The use of a thickness-shear mode resonator to monitor the formation ofbarium sulphate in samples of produced water collected at the well headhas been described. See, Emmons, D. H. and Jordan, M. M., “Thedevelopment of near-real time monitoring of scale deposition”, SPEOilfield Scale Symposium, Aberdeen, 27-28 Jan. 1999 (hereinafter “Emmonsand Jordan”). The resonator was immersed in a fixed volume of producedwater and known amounts of soluble barium ions were added to precipitatebarium sulphate scale. The resonator detected the formation of scale onits sensing surface by a decrease in resonant frequency. The amount ofbarium added before scale formation was detected by the resonator gavean indication of the level of inhibition in the produced water. Emmonsand Jordan argued that the formation of scale by small additions ofbarium ions indicated the produced water was close to scaling andtreatment of the well by a suitable scale inhibitor was required. Notethat this method of monitoring scale formation is not an in situ methodand does not measure the spontaneous formation of scale under downholeconditions of temperature, pressure, composition and flow. In addition,the resonator was not able to clean the scale from its sensing surface.A quartz crystal microbalance to monitor the formation of calciumcarbonate scale under laboratory conditions has been described. See,Gabrielli, C., Keddam, M., Khalil, A., Maurin, G., Perrot, H., Rosset,R. and Zidoune, M., “Quartz crystal microbalance investigation ofelectrochemical calcium carbonate scaling”, J. Electrochem. Soc., 145,2386-2395 (1998). The resonant frequency of the microbalance was 6 MHzand calcium carbonate deposition rates of 200-400 μg/cm² per hour weremeasured. The calcium carbonate scale was observed to be the mineralcalcite, which, with an assumed density of 2.71 g/cm³, gave deposits of0.7-1.5 μm in thickness. The rate of scale accumulation measured by thequartz microbalance was compared with a standard electrochemical scalemonitor that measured the redox current passing through an electrode aswater was reduced. The decline in redox current gave an indirect measureof the decrease in the surface area of the electrode as it was coveredwith scale and was observed to be less sensitive to scale formation thanthe quartz crystal microbalance. The use of a piezoelectric quartzcrystal to monitor the fouling of surfaces in a water cooling tower byinorganic scale and bacterial growth at ambient conditions has beendescribed. See, Nohata, Y. and Taguchi, H., “An ultrasensitive foulingmonitoring system for cooling tower”, Materials Performance, 34, 43-46(1995) (hereinafter “Nohata and Taguchi”). Although Nohata and Taguchidid not specifically disclose the operating frequency of the quartzcrystal, a value of about 5 MHz can be deduced from the measuredaccumulation rates of 1-20 μg/cm² per day.

The use of a tuning fork for measuring the deposition of scale in asurface process system has been disclosed. See, U.S. Pat. No. 5,969,235.The accumulation of scale on the tines of the tuning fork causes a shiftin the characteristic vibrating frequency of the tuning fork as measuredby a suitable electronic device, such as a piezoelectric cell. Thechange in vibrating frequency of the tuning fork, indicating thedeposition of scale, was used to control the addition of scale inhibitorto the process stream.

Non-acoustic scale sensing techniques have also been reported. A methodof determining the accumulation of scale in petroleum pipelines using aheat transfer sensor has been described. See, U.S. Pat. No. 4,718,774.The scale formed on the external wall of the sensor impeded the loss ofheat from a heating element in the sensor to the fluid flowing in thepipeline. The decrease in heat flow was measured by means of atemperature sensor. A wellbore scale monitor that measured theradioactivity of the radium salts precipitated with other alkaline earthmetal salts has been described. See, U.S. Pat. No. 4,856,584(hereinafter “Sneider”). Sneider discloses the use of measurements ofscale radioactivity to indicate when and where the placement of scaleinhibitor is required. Another scale monitoring technique is disclosedin U.S. Pat. No. 5,038,033; the radioactivity of the scale was detectedby a wireline gamma ray detector, correcting for the natural gammaradiation emitted from the surrounding rock formations.

Accurately measured pressure drops over various sections of areinjection pipeline in a geothermal power plant has been used tomonitor the growth of silica scale. See, Stock, D. D., “The use ofpressure drop measurements to monitor scale build-up in pipelines andwells”, Geothermal Resources Council Trans., 14, 1645-1651 (1990). Themeasured pressure drops across the sections of pipe produced frictionfactors in the range 0.1-0.2, compared to an expected value of 0.01.Cleaning the silica scale from the pipeline sections using a wire brushpig resulted in the friction factor dropping below a value of 0.06. Bothlaboratory and field systems to evaluate the scaling potential ofoilfield brine samples by monitoring the pressure drop across acapillary tube through which the brine flows and deposits scale arebeing currently produced by the company Oilfield Production AnalystsLtd. (see product brochures for P-MAC 2000, 3000 and 4000 series). Threeoptical techniques to monitor fouling in industrial process systems weredescribed in Flemming, H-C., Tamachkiarowa, A., Klahre, J. and Schmitt,J., “Monitoring of fouling and biofouling in technical systems”, WaterSci. Tech., 38, 291-298 (1998). The techniques consisted of measurementof the intensity of light reflected from a small optical fibre probe,the measurement of turbidity through optical windows in a flow line(using periodically cleaned windows in the flow line as a referenceoptical pathlength) and an infrared spectroscopy flow cell. Theaccumulation of deposits and the chemical nature of the deposits on theoptical windows of the infrared flow cell could be determined from theinfrared spectra.

A number of published reports have described the application of sonicenergy for cleaning producing oil wells and equipment in similarindustrial processes. A method of cleaning downhole deposits, such astar, from producing formations and production tubing was disclosed inU.S. Pat. No. 3,970,146. However, no details were given of the power orfrequency of the sound used for wellbore cleaning. A low frequency(20-100 Hz) vibrating device for cleaning deposits on the walls ofcasing and tubing and in formations and gravel packs was disclosed inU.S. Pat. No. 4,280,557. The vibrations were generated in the device byan orbiting mass on an unbalanced rotor, which, in turn, produced awhirling vibratory pressure of large amplitude in the fluid in theannulus. U.S. Pat. No. 4,320,528 disclosed a method of removing ironoxide corrosion products and other scaling deposits from the pipes ofsteam generators using a combination of high power sound and ahigh-temperature solvent (e.g., sodium EDTA, citric acid and a corrosioninhibitor). The acoustic transducers operated in the frequency range2-200 kHz and generated an output acoustic power greater than 0.2 W/cm²,a value which is above the cavitation threshold of aerated water atambient pressure and temperature. See, Esche, R., “Untersuchung derSchwingungkavitation in Flüssigkeiten”, Acustica, 2, AB208-218 (1952);Mason, T. J. and Lorimer, J. P., Sonochemistry: Theory, Applications andUses of Ultrasound in Chemistry, p 31, Ellis Horwood, Chichester, UK(1988). The transducers were located permanently on the outside of theheat exchanger tubes. U.S. Pat. No. 4,444,146 disclosed an ultrasonicmethod to clean the fouled surfaces of submerged structures, such as thehulls of ships. The ultrasonic cleaner consisted of two ultrasonictransducers focussed on a small area of surface to be cleaned. Thetransducers operated at slightly different frequencies, typically in therange 180-210 kHz; no details were disclosed on the acoustic powerrequired to clean the fouled surfaces. UK Patent Application 2 165 330 A(hereinafter “D'Arcy et al.”) disclosed a method of cleaning underwaterstructures to depths of up to 1000 meters using focussed ultrasound inthe frequency range 40-100 kHz. The ultrasound was generated andfocussed using an array of transducers located on the concave surface ofa spherical cap. The density of acoustic power at the focal point of thearray of transducers was stated to be about 500 W/cm², a value that isapproximately 3 orders of magnitude above the cavitation threshold ofwater at ambient pressure. D'Arcy et al. suggested the high poweracoustic array could be used to clean the base of oil productionplatforms. U.S. Pat. No. 5,184,678 disclosed the design of a high poweracoustic logging tool to stimulate fluid production from oil wells. Theacoustic power was provided by pulsed magnetostrictive transducersoperating in the frequency range 5-30 kHz and emitting an acoustic powerdensity of up to 1 W/cm². The tool was designed to give a stand-off fromthe treated formation of 0.2-0.5λ, where λ is the wavelength of thesound in the borehole fluid. The treated formations were exposed to theacoustic power for periods of 5-60 minutes. According to U.S. Pat. No.5,184,678, the applied ultrasound reduced the viscosity of the fluid inpermeable formations and fluidised the particulate matter, thusfacilitating its removal.

It has been shown that ultrasound applied at a frequency of 10 kHz couldremove asphaltene deposits from a sand pack saturated with both waterand kerosene at ambient pressure. See, Gollapdi, U. K., Bang, S. S. andIslam, M. R., “Ultrasonic treatment for removal of asphaltene depositsduring petroleum production”, SPE International Conf. Formation DamageControl, Lafayette, La., February 1994, SPE 27377. Although the acousticpower applied to the sand packs during cleaning was not measured, theultrasonic transducer could generate a maximum output acoustical powerof 250 W. The authors discussed the role of acoustic cavitation in thecleaning process and acoustic cavitation was undoubtedly achieved at thepower settings reported. The asphaltene deposits were observed to beremoved significantly more efficiently by the ultrasound in kerosenethan in water. It has been demonstrated under laboratory conditions thatthe damage caused to permeable formation by the invasion of clayparticles from drilling fluids can be partially removed by theapplication of high power ultrasound. See, Venkitaraman, A., Roberts, P.M. and Sharma. M. M., “Ultrasonic removal of near-wellbore damage causedby fines and mud solids”, SPE Drilling & Completions, 10, 193-197(1995). Two ultrasonic transducers were used; one was a high powerultrasonic horn operating at a frequency of 20 kHz with an output powerof up to 250 W and the other was a low power transducer operating overfrequency range 10-100 kHz. The same authors subsequently evaluated theapplication of high power ultrasound under laboratory conditions for theremoval of organic deposits and formation damage caused by the invasionof drilling fluid filtrate containing water-soluble polymers. See,Roberts, P. M., Venkitaraman, A. and Sharma, N. M., “Ultrasonic removalof organic deposits and polymer induced formation damage”, SPE FormationDamage Control Symp., Lafayette, La., February 1996, SPE 31129. Usingthe same ultrasonic transducers, it was demonstrated thatpolymer-induced formation damage was considerably more difficult toremove than the damage caused by clay fines. However, formation damageresulting from the precipitation of wax in the test core samples couldbe removed by sonication when the core samples were soaked in a suitablesolvent.

U.S. Pat. No. 5,595,243 disclosed the use of a general purpose acousticcleaning tool for improving the near wellbore permeability of producingformations by redissolving or resuspending restricting materials. Thecleaning tool was reported to generate acoustic power densities of up to2 W/cm², which is above the cavitation threshold for water at ambienttemperature and pressure. See, U.S. Pat. No. 4,280,557. The tool, whichconsisted of an array of air-backed high power acoustic transducers ofthe type described by Widener, was designed to be deployed into the wellon a wireline cable. See, Widener, M. W., “The development ofhigh-efficiency narrow-band transducers and arrays”, J. Acoust. Soc.Amer., 67, 1051-7 (1980); Widener, M. W., “The development of a deepsubmergence air-backed transducer”, J. Acoust. Soc. Amer., 80, 1852-3(1986). The transducers described by Widener would be expected tooperate in the frequency range 10-100 kHz. U.S. Pat. No. 5,676,213disclosed the use of high power ultrasound to remove the filter cakeformed by the drilling fluid during the drilling of a well in order tomeasure the pressure in permeable formations. The high power ultrasoundwas generated by a focussing transducer operating in the frequency range100-500 kHz and capable of operating at a peak input power of up to 1kW. U.S. Pat. No. 5,727,628 disclosed an ultrasonic tool for cleaningproducing wells. The wireline-deployable tool consisted of an array ofmagnetostrictive transducers operating in the frequency range 18-25 kHz(preferably at 20 kHz) and emitting an acoustic power density in therange 8-12 W/cm². The tool was also equipped with a pump to remove thedebris of the fouling deposits disaggregated by the ultrasonic tool.U.S. Pat. No. 5,735,226 disclosed a method to prevent the fouling ofships and other marine structures by the use of ultrasound over thefrequency range 20-60 kHz. One demonstration of the technique was thelocation of a number of ultrasonic transducers on the hull of a shipover a period of 4 months. Over this time period the transducers, whichwere powered intermittently, gave effective relief from marine fouling.U.S. Pat. No. 5,735,226 revealed no details of the power consumption ofthe transducers, but one embodiment of the invention consisted of thearray being powered by a 9 volt battery. U.S. Pat. No. 5,889,209disclosed the use of high power ultrasound to prevent biofouling ofchemical sensors used in aquatic environments. The ultrasound wasgenerated by a transducer operating in the frequency range 10-100 kHzand yielding a sufficient power density (>0.1-1 W/cm²) to drive acousticcavitation. U.S. Pat. No. 5,889,209 disclosed the use of the acousticcleaning technique to maintain the performance of a dissolved oxygensensor located in microbiologically active water for seven days. Thetransducer was located over the range 4-10 mm from the active membraneof the oxygen sensor and activated for a time period of 6-90 secondsover a time interval of 5-120 minutes.

Several papers and patents have reported on the use of high powerultrasound to accelerate the dissolution of scale by chemical scaledissolvers applied in pipelines and producing oil wells. Paul, J. M. andMorris, R. L., “Method for removing alkaline scale”, InternationalPatent Application WO 93/24199, 9 Dec. 1993 describes the use of lowfrequency (1.5-6.5 kHz) sonic energy to accelerate the dissolution ofalkaline earth metal scales using scale dissolving solutions (typicallycontaining the chelating agents EDTA or DTPA). Gunarathne, andGunarathne and Keatch have shown that the application of low-powerultrasound can increase the rate of dissolution of barium sulphate scaleusing commercially available scale dissolvers; the power density wasclaimed to be below that required to cavitate the scale dissolvingsolution.

In conclusion, there appears to be no prior art that teaches or suggestseither an acoustic scale sensor or an acoustic cleaning device locatedpermanently or quasi-permanently in a well producing hydrocarbons. Thereappears to be no prior art of teaches or suggests the concept of asensor for hydrocarbon wells to monitor the formation of inorganic ororganic scales, biofouling or corrosion and initiate a cleaning action.Additionally, there appears to be no prior art that teaches or suggestsan on-line deposits monitoring and cleaning device located on thesurface facilities of a producing oil well using an ultrasonictransducer operating in its longitudinal mode and coupled to theproduced fluids using a coupling material, such as an acoustic horn, towhich the deposits adhere.

SUMMARY OF THE INVENTION

It is an object of this invention to describe an apparatus that can beplaced at various locations to monitor the deposition of scale and otherdeposits and preferably to remove such deposits.

According to the invention a deposit monitoring apparatus located in ahydrocarbon wellbore is provided comprising: an acoustic device adaptedto operate in a resonance mode including a monitoring surface directlyexposed to fluids in a hydrocarbon wellbore, wherein the deposition ofmaterial on the monitoring surface is monitored by measuring a change inresonance frequency of the acoustic device; and a power supply adaptedto supply said monitor with electrical energy.

The acoustic device is preferably mounted either permanently orquasi-permanently in the wellbore.

According to another aspect of the invention, a deposit monitoringapparatus located in a hydrocarbon wellbore is provided, comprising: adeposit monitor adapted to measure deposition of material on amonitoring surface that is directly exposed to fluids in the hydrocarbonwellbore; a power supply adapted to supply said monitor with electricalenergy; and a deposit removal system in communication with the depositmonitor adapted to at least partially remove the deposition from themonitoring surface, the deposit removal system being in a control loopwith said deposit monitor.

In a preferred embodiment the deposit monitor is a high power ultrasonictransducer, operating in a longitudinal mode, coupled to the fluidsproduced from the well by a solid coupling device, such as an acoustichorn, for measuring the deposition of the organic and inorganic scalesthat form from the fluids produced in hydrocarbon wells. By longitudinalmode we mean that the surface of the device exposed to the fluid is movepredominantly normal rather than parallel to the exposed surface.

The transducer is preferably designed to operate in the frequency range10-250 kHz with a maximum acoustical power output of 10-500 W. Themaximum power is preferably only used during the cleaning process forless than 10 seconds or even less than 1 second. More preferably thefrequency range is between 10 kHz and 150 kHz. The optimal frequencyrange is believed to lie within the range of 50 to 100 kHz. It was foundthat higher frequencies, particularly frequencies in the MHz region, arenot applicable for permanent downhole deployment.

The growth of wellbore deposits is monitored by the decrease in theresonant frequency of the transducer with increasing thickness ofdeposit. The resonant frequency is conveniently determined from the realpart of the admittance spectrum of the transducer. The horn amplifiesthe effect of the scale deposit on the measured admittance of thetransducer.

The term “horn” is used as a generic expression for a solid couplingdevice, sally consisting of a tapering end piece mounted onto the bodyof the acoustic transducer. The horn can be bolted or glued to the bodyof the transducer material. The length of the horn is preferably someinteger multiple of half the wavelength of the acoustic wave generated.The tapering can be achieved by cutting steps into the material orsmoothly, e.g., as exponential tapering.

The horn material can be chosen arbitrarily from a large variety ofsolid materials. It is however a preferred feature of the invention tohave the horn tip made of a material that matches the properties of thesurrounding material, so as to ensure that the deposits on thesurrounding material are accurately measured.

At least the tip of the horn is directly exposed to the wellbore fluidcausing the deposition.

The transducer is able to detect inorganic deposits or scales, such asbarium sulphate or calcium and organic deposits, such as waxes, at athickness of approximately 100 μm.

Scales can be removed from the transducer by operation at high power;the transducer is therefore able both to detect and clean deposits. Theultrasonic transducer is also able to clean scale from the surfaces ofother components located at various positions in producing hydrocarbonwells, such as those made of glass, metal or plastics. The acoustictransducers can be permanently located in producing hydrocarbon wellsand indicate the location and rate of scale deposition. The measurementof scale deposition can be used to direct scale treatment procedures,such as the placement of inhibitors or the application of scaledissolvers, and to warn of scale accumulation on critical downholeproduction equipment, such as chokes, sliding sleeves and separators.The acoustic scale sensor can also be used to determine theeffectiveness of any treatment to remove scale or other wellboredeposits.

The control loop comprises either be a suitably programmedmicroprocessor or computer executing pre-programmed tasks. Or, it couldinclude visual display units allowing a human operator to make decisionsbased on the measurements provided by the deposit monitor.

Permanently or quasi-permanently installed in a wellbore or below thesurface of the Earth refers to installation that are fixed to thewellbore and thus can be retrieved to the surface only under great costsand even sacrifice of equipment.

These and other features of the invention, preferred embodiments andvariants thereof, possible applications and advantages will becomeappreciated and understood by those skilled in the art from thefollowing detailed description and drawings.

DRAWINGS

FIGS. 1A-D are schematic illustrations of examples of acoustic devices,according to certain embodiments of the invention;

FIG. 2A shows the resonant frequencies of an acoustic device shown inFIG. 1B as measured by the real part of the admittance in air, water andkerosene;

FIG. 2B shows an enlarged part of FIG. 2B;

FIG. 3 illustrates the shift of the resonant frequency with increasingscale deposition at a hydrostatic pressure of 272 bar (4000 psi);

FIG. 4 illustrates the shift of the resonant frequency with increasingwax deposition at ambient pressure;

FIG. 5 illustrates the shift of the resonant frequency after cleaningthe tip of a sensor;

FIG. 6 illustrates the power supply used to remove deposits from asurface exposed to wellbore fluids;

FIG. 7 illustrates a downhole installation in accordance with theinvention;

FIG. 8 illustrates a sensor installation with self-cleaning equipment inaccordance with the invention;

FIGS. 9A,B illustrate further sensor installations with self-cleaningequipment in accordance with the invention; and

FIG. 10 shows a schematic sensor adapted to further analyse wellboredeposits.

MODE(S) FOR CARRYING OUT THE INVENTION

(I) Acoustic Scale/Deposits Sensor

The scale (or deposits) sensor consists of an ultrasonic piezoelectrictransducer operating in a longitudinal mode coupled to a suitable metalhorn. The resulting ultrasonic device is characterised by a sharpresonant frequency, which can be conveniently determined by themeasurement of the admittance (or impedance) spectrum of the device. Theresonance frequency of the appropriate longitudinal mode is sensitive toany solid deposit that forms on the tip of the horn and the magnitude ofthe frequency shift is a measure of the mass loading. These ultrasonictransducers typically operate in the frequency range 10-100 kHz and candeliver high levels of acoustic power, typically in the range 1-500 W,when driven by a high input alternating voltage at its resonantfrequency.

FIG. 1 shows schematics of several types of acoustic horn attached to anultrasonic transducer. The basic elements of the scale deposit sensor 10are a transducer 11 made of piezoelectric material, a power supply withelectrodes 12 to cause oscillations of the transducer, and a horn 13.The horn is made of aluminum. The working surface of the transducer 131,on which the deposition of the scale is sensed, is the tip of the horn.In a downhole and surface installations, the tip 131 (or 141, 151, and161 for the embodiments of FIGS. 1B, 1C, and 1D respectively) will beexposed to wellbore/production fluids and accumulate deposits.

The resonant frequency of the acoustic device operating in alongitudinal mode is determined by the size of the piezoelectric and theattached horn and the materials from which the piezoelectric and metalhorn are constructed. The variants differ in their respective horndesign. The horn 13 shown in FIG. 1A has a stepwise tapering. In FIG.1B, the tapering is smooth with an exponentially reducing diameter ofthe horn 14. The horn coupled to the ultrasonic piezoelectric transduceris made of aluminium. The resulting device has a sharp resonantfrequency in air of 40 kHz nominally and the area of the horn tip was0.2 cm². In FIG. 1C, the tapering is degenerated to a single step givingthe horn 15 a pin-like shape. Other horn shapes can be envisaged,including the case where its thickness is very much less than thewavelength of sound and the horn is a thin layer of material thatcouples the ultrasonic transducer to the bore hole fluids and theirdeposits.

Further indicated in FIG. 1 is the length of the horn as integermultiple N of half the wavelength (λ/2) of the acoustic wave generatedby the transducer. FIG. 1D shows the case where the length of the horn16 is very much less than λ/2 and the horn tip 161 has the same area asthe ultrasonic transducer.

Table 1 shows the resonant frequency of the ultrasonic horns and thewidths of the resonance at half peak height shown in FIG. 1A-C. The peakwidth of the resonance is typically 1-2% of the resonant frequency.

TABLE 1 Resonant frequencies in air of the ultrasonic transducers shownin FIG. 1A-C. Width of resonace Transducer type in Resonant frequency athalf peak FIG. 1 (kHz) height (Hz) A 19.86 21 B 39.04 52 C 54.63 96

FIG. 2 shows the resonant frequency of the acoustic sensor shown in FIG.1B as measured by the real part of the admittance in Siemens (S). Theresonant frequency of the ultrasonic transducer is modified by thenature of the fluid in which it is immersed. FIG. 2 compares theresonant frequency of the ultrasonic transducer immersed in air, waterand kerosene. The resonance frequency of the transducer decreases whenthe metal horn is immersed in a denser fluid and the resonance broadenswith more viscous fluids. The fine structure (shown in FIG. 2B) on theadmittance spectra measured when the transducer is immersed in water andkerosene at ambient pressure is caused by the attachment of small airbubbles to the horn.

FIG. 3 shows the shift in the resonant frequency of a transducer andmetal horn with thickness of attached scale when the horn is immersed inwater in a pressure vessel at a hydrostatic pressure of 4000 psi (270bar). The resonant frequency of the 20 kHz transducer shifts by 85 Hzper millimeter of scale of density 4.50 g/cm³.

Further measurements show the variation of the shift in the resonancefrequency of a 40 kHz transducer as a function of scale thickness inwater at ambient pressure for two scale types with densities of 2.75 and4.50 g/cm³. The resonant frequency of the transducer decreases by 774 Hzper millimeter of scale of density ρ=2.75 g/cm³ and 966 Hz permillimeter of scale of density ρ=4.50 g/cm³.

The accumulation of inorganic scale can also be detected by a shift inthe resonant frequency of a transducer when the horn is immersed in aliquid hydrocarbon. When comparing the admittance spectra of a 40 kHztransducer and horn immersed in kerosene at ambient pressure with andwithout inorganic scale attached, it was found that the shift in theresonant frequency of the transducer is 1317 Hz per millimeter of scaleof density ρ=4.50 g/cm³ in kerosene at ambient pressure.

The resonant frequency of an ultrasonic transducer and horn alsodecreases when organic deposits, such as wax, form on the tip of thehorn. FIG. 4 shows the admittance spectra of a 40 kHz ultrasonictransducer and metal horn immersed in water at ambient pressure with waxof various thicknesses attached to the tip of the horn. The resonantfrequency of the transducer decreases by 180 Hz per millimeter of wax ofdensity 0.79 g/cm³ when the horn is immersed in water at ambientpressure.

(II) Acoustic Scale/Deposits Cleaning

The inorganic and organic deposits that accumulate on the tip of thehorn can be removed by applying a high alternating voltage to thetransducer. The strain produced in the transducer and the attached hornbreaks the bond between the horn tip and the deposit and the tip iscleaned. The acoustic deposits sensor can therefore be self-cleaning.The acoustic scale/deposits sensor is also able to clean scale from thesurfaces of other components, which are either in close proximity to thetip of the horn or incorporated into it.

The scale was deposited on the tip of a metal horn connected to a 20 kHzultrasonic transducer. The horn and transducer were housed in the cap ofa cell that can be pressurised to 340 bar (5000 psi). The area of thehorn tip is 2.87 cm², the average scale thickness is 1.47 mm and thescale density is 4.50 g/cm³. A high voltage was applied to thetransducer and the horn was sonicated in a pressure cell at ahydrostatic pressure of 136 bar (2000 psi). All of the inorganic scalewas removed from the tip of the horn.

FIG. 5 compares the admittance spectra of the acoustic transducer andmetal horn before and after acoustic cleaning at 136 bar (2000 psi). Theresonant frequency of the ultrasonic transducer and metal horn changedby 210 Hz after sonication.

The electrical power that is required by the transducer to remove thescale from the tip of the horn can be determined from the input voltageand current. The frequency of the input voltage and current is 19.84kHz, which compares to a value of 19.81 kHz obtained from the admittancespectrum of the cleaned horn. The scale, which consisted of a depositwith a density of 4.50 g/cm³ and a thickness of 1.47 mm, was removedfrom the metal horn at a hydrostatic pressure of 136 bar (2000 psi).FIG. 6 shows the input power waveform supplied to the transducer duringthe cleaning of the scale. The modulus of the time-averaged inputelectrical power of the waveform shown in FIG. 6 is 20.4 W.

(III) Some Applications of an Acoustic Deposits Sensor and Cleaner

According to the invention a permanent scale/deposits monitor isprovided. Ultrasonic transducers, of the type described above, operatingover a range of frequencies, typically 10-150 kHz, can be permanentlyinstalled in a producing hydrocarbon well to determine the rate ofaccumulation of scale and other deposits (wax, asphaltene, etc.). Thetransducers can be deployed at various locations in the well, includingsurface production facilities such as subsea valves, risers, separatorsand any associated pipes and conduits, in order to determine thelocation of scale formation. The transducers can be used to determinethe rate of accumulation of scale and other deposits and thesemeasurements can be used to select the appropriate treatment to maintainthe productivity of the well, such as the deployment of scale inhibitorsor scale dissolvers. The deposits accumulated on the horn attached tothe transducer can be cleaned periodically to maintain its sensitivity.

According to the invention the apparatus can be used in the maintenanceof intelligent completions. More specifically, the ultrasonic sensor canbe used to determine the accumulation of scale, wax or asphaltene oncritical downhole equipment, such as chokes, valves and the slidingsleeves on intelligent completion systems. The measurements can be usedto select an appropriate treatment to maintain the integrity andoperation of the equipment, e.g., deployment of a scale dissolver toremove scale from critical components.

In FIG. 7 reference is made to an “intelligent” completion systemdeployed to control the flow of wellbore fluid into a production pipe.

FIG. 7 shows a schematic of part of an intelligent completion system. Anintelligent completion allows active control of downhole processes suchas flow rate measurements and control. A wellbore 70 is shown with acasing 701 installed. At certain locations the casing 701 is perforatedby holes 702 to allow wellbore fluid to enter the wellbore. Installedwithin the wellbore is a production pipe 71 with a slotted section 711.Wellbore fluid enters the production pipe via the slotted section 711.Mounted within the production pipe is a sliding sleeve 712. The slidingsleeve 712 controls the flow of reservoir fluids into the productiontubing from a particular section of the well. The flow rate, pressureand temperature of the production fluids are measured by a sensor sub713 near the slotted section 711 into the tubing. The sliding sleeve 712is operated by a downhole electrical motor 714 that is powered by meansof an electrical cable 721 from the surface 72.

One problem associated with the use of a sliding sleeve is the formationof wellbore deposits, such as inorganic scale or wax, on the sleeve orits track. It is common practice to design the completion hardware suchthat the electric motor 714 is sufficiently powerful to remove smallamounts of scale when the sleeve is moved. However, attempting to removescale or other deposits by scraping with the sleeve 712 may result indamage to the track and possible jamming. The location of an acousticscale sensor 715 close to the sliding sleeve 712 will enable theaccumulation of deposits on completion hardware to be assessedquantitatively through control equipment 722 located at the surface.Transfer of energy and data signal between control equipment 722 and thesensor 715 is done via wiring 716 run with the production tubing. Ifscale accumulation on the sleeve is considered to be above a level atwhich it can be safely operated, then an external scale removal processmay be required (e.g., application of a scale dissolver solution).

Alternatively, the control equipment 722 could be used to activate thesleeve periodically or responsive to a threshold amount of deposit asmeasured by the scale sensor 715. In yet another alternative the controlequipment 722 could at least partly be incorporated into the downholeinstallation. By thus providing a direct feedback control between sleeveand deposit monitor 715, the amount of intervention from the surface canbe reduced.

According to the invention, acoustic cleaning of scale and otherdeposits is provided. The ultrasonic transducer and associated horn canbe used to remove scale and other deposits (wax, asphaltene, etc.) fromcritical components that are used in measurement devices exposed towellbore/production fluids in the wellbore or on the surface. Thecomponents can be part of measurement systems that are eitherpermanently installed in producing hydrocarbon wells or surfacefacilities or temporarily exposed to the fluids produced by thehydrocarbon well, e.g., on a wireline-deployable tool. Criticalcomponents exposed to wellbore fluids that may require cleaning includeseparation membranes, optical windows and electrical contacts such aselectrodes.

FIG. 8 refers to a venturi-type flowmeter having a gamma-ray sensor inits constriction section.

It shows a schematic of a gradio-venturi tube 81 located in the tubing80 of a producing hydrocarbon well. The section of tubing 80 can belocated either in a well or on surface facilities. Located within theconstriction formed by the venturi 81 is a gamma ray sensor 82. Detailsof those known components of a gradio-venturi can be found for examplein U.S. Pat. No. 5,591,922. The gamma ray sensor, which consists of agamma ray source 821 (frequently a dual energy source) and a gamma raydetector 822, such as a photomultiplier tube. The gamma rays enter andleave the tube by means of nuclear windows 823 made of a material suchas boron carbide. Gamma-ray devices as described above are known as suchand described for example in the U.K. Patent Application No. 9919271.8.

A significant problem faced by the gamma ray density measurement is theaccumulation of inorganic or organic scales on the nuclear windows 823.For example, the accumulation of small amounts of barium sulphate(barite) scale may result in a serious overestimate of the density ofthe production fluids. Similarly, the accumulation of radioactive scaleson the nuclear windows may give rise to erroneous measurements. Theaccumulation of organic scales, such as asphaltenes that may containelements with of high atomic number, can also corrupt the densitymeasurement.

One solution to the problem of deposits accumulation on the nuclearwindows 823 is to incorporate them into a high power ultrasonictransducer and horn as is the subject of the present invention (see FIG.1 above). FIG. 8 shows the two windows located in a hollow horn 824 andtransducer 825; the windows 823 are cleaned by operating the transducerin its high power mode. The accumulation of deposits on the windows canbe measured by a shift in the resonance frequency of the transducer andhorn, as described above.

The gradio-venturi tube of the type above can be located either on thesurface or downhole, e.g., as part of an intelligent completion system.

FIG. 9 shows further examples of the use of an ultrasonic transducer andhorn to clean measurement devices that may be deployed in a producinghydrocarbon well or at surface installation monitoring the flow ofhydrocarbon from the well.

FIG. 9A shows a schematic of an optical window 931 located at the tip ofa horn 93 attached to an ultrasonic transducer 91. The optical window931, which can be made of a suitably resistant material such as diamondor sapphire, can be connected to a source and detector using an opticalfibre 95 or other optical conduit such as a light pipe. The opticaltransmission of the window can be maintained by removing deposits oforganic and inorganic scales with ultrasonic cleaning. The accumulationof deposits on the optical window can be determined by either thedecreased transmissivity of the window or the change in the admittancespectrum of the transducer 91 and horn 93 assembly. The optical windowscan be located either in a producing oil well, on a permanent ofquasi-permanent basis, or on the surface facilities. The acousticcleaning of optical windows can also be applied to wireline loggingtools, such as the optical windows used in the Optical Fluid Analyser inSchlumberger's Modular Dynamics Formation Tester tool (described in U.S.Pat. No. 4,994,671) or the windows used in the optical probes to monitormultiphase production from hydrocarbon wells, as described in U.S. Pat.No. 5,831,743.

FIG. 9B shows a schematic of an ultrasonic transducer 91 and horn 94used to clean the membrane 961 of an ion selective electrode 96 thatcould be used to measure the activity of an ionic species in the waterproduced from a hydrocarbon well. The performance of the membrane 961 ismaintained by the cleaning action of the ultrasonic transducer 91 andhorn 94, which are in close proximity to the membrane. The ultrasoniccleaning technique can be used to maintain the permeability of membranesused to separate various components from the fluids produced fromhydrocarbon wells, e.g., gas extraction membranes, oil-water separationmembranes. Note that a separation membrane and electrode can also beincorporated into the horn coupled to a transducer. The separationmembranes can be similarly located in a producing wellbore or in thesurface facilities of a producing well.

According to the invention, a method for the identification of scaletype is provided. The resonant frequency of the transducer and hornresponds to the mass of the attached deposits and it may not be possibleto identify the type of scale deposit in the absence of othermeasurements. One of a variety of measurements could be used to identifythe type of scale deposit accumulated on the acoustic horn and thusenable acoustic scale sensor to measure the thickness of the deposit.Two examples are give. Firstly, a spectroscopic measurement could beemployed, using an optical widow of the design shown in FIG. 9A. Thescale type can identified by a total internal reflection spectroscopictechnique, such as infrared spectroscopy, or Raman spectroscopy, which,as reflection and scattering techniques, are insensitive to thethickness of the scale layer. A second possible technique is to use asecond acoustic sensor, embedded in the acoustic horn, to measure theacoustic transit time through the scale deposit.

FIG. 10 shows a schematic of the acoustic sensor used to measure theacoustic transit time ΔT_(t) through the scale film. This acousticsensor could be used either downhole or above ground on the surfacefacilities. Pulsed ultrasound is emitted from an emitting transducer 981and the reflections are collected by a second receiving transducer 982.The acoustic impedance Z of the scale layer 99 can be estimated by themeasured values of the mass W of scale attached to the acoustic horn 97and ΔT_(t). The thickness of scale h attached to the horn is related toW by $\begin{matrix}{h = \frac{W}{\rho_{s}A}} & \lbrack 1\rbrack\end{matrix}$where ρ_(s), is the density of the scale and A is the area of the tip ofthe horn. The thickness h is related to ΔT_(t) by $\begin{matrix}{{h = \frac{\Delta\quad T_{t}V}{2}},} & \lbrack 2\rbrack\end{matrix}$where V is the velocity of sound in the scale layer. The combination ofeqn. [1] and [2] gives $\begin{matrix}{Z = {{\rho_{s}V} = {\frac{2\quad W}{\Delta\quad T_{1}A}.}}} & \lbrack 3\rbrack\end{matrix}$

The scale type can be discriminated by the measured value of Z. Table 4shows the values of the acoustic impedance Z as a function of scaletype. When Z is identified the thickness h of the scale deposit can becalculated using eqns. [1] or [2] using the known values of V and ρ_(s).

TABLE 4 Values of the acoustic impedance Z for various inorganic scales.Scale type Acoustic impedance Z (kg/m²s × 10⁻⁶) calcite 17.6 anhydrite18.0 celestite 19.1 barite 19.9

According to the invention monitoring scale/deposits removal techniquesare provided. The acoustic scale sensor can be used to evaluate theefficiency of an external scale removal treatment process, such as ascale dissolver solution or a physical scale removal technique. Thescale removal treatment can be monitored in real time using the changein the resonant frequency of the transducer and horn. The acoustic scalemonitor can therefore be used as part of a scale maintenance servicethat monitors scale accumulation and evaluates the cleaning/removalprocess. The maintenance service can be applied to the accumulation ofscale both in a producing wellbore or in the surface equipment.

According to the invention, fluid composition and pressure effects canbe determined. The resonant frequency of the ultrasonic transducer andhorn with no deposits attached depends on the composition of the fluidwith which it is in contact and its hydrostatic pressure. A change inthe composition of the produced fluid or its hydrostatic pressure maytherefore obscure the change in resonant frequency due to theaccumulation of inorganic or organic deposits. A change in resonantfrequency due to a change in fluid properties can be measured using asecond matched transducer and horn, which is in close proximity to thescale detector. The second transducer is cleaned frequently to ensurethat the horn is always free from inorganic and organic scales and thatthe changes in its resonant frequency are solely caused by changes influid composition and pressure.

Surface Modification of Acoustic Horn. An important aspect of ascale/deposits sensor permanently installed in a producing hydrocarbonwell or in the surface facilities is that the rate of accumulation ofdeposits must be the same as that of the region of the wellbore it ismonitoring. The composition and morphology of the surface of the tip ofthe horn should be such as to give the same rate of scale accumulationas the solid surfaces in the close environs of the sensor. The surfaceof the horn tip should be suitably controlled by the choice of materialand/or coating. Th material and coating used to fabricate the hornshould be able to withstand repeated sonication.

Size of Transducer and Horn. The size of an ultrasonic transducer andhorn may be a critical issue for the permanent placement of such adevice for scale/deposits monitoring. The size of the transducer andhorn are largely determined by the desired frequency of operation andthe materials use to them. High power ultrasonic transducers and hornsin the size range 5-6 cm, which operate in the frequency range 89-113kHz, have been reported by Lal and White. See, Lal, A. and White, R. M.,“Silicon microfabricated horns for power ultrasonics”, Sensors andActuators, A54, 542-546 (1996). The acoustic horns were microfabricatedfrom silicon wafers.

While preferred embodiments of the invention have been described, thedescriptions and examples are merely illustrative and are not intendedto limit the present invention.

1. A deposit monitoring apparatus located in a hydrocarbon wellborecomprising: an acoustic device for operating in a resonance mode whichis longitudinal including a monitoring surface directly exposed tofluids in a hydrocarbon wellbore, wherein the deposition of material onthe monitoring surface is monitored by measuring a change in resonancefrequency of the acoustic device; and a power supply for supplying saidacoustic device with electrical energy.
 2. The apparatus of claim 1,wherein the acoustic device is mounted either permanently orquasi-permanently in the wellbore.
 3. The apparatus of claim 1, whereinthe acoustic device further comprises a transducer, and a focussingelement coupled to the transducer.
 4. The apparatus of claim 3, whereinthe focussing element is an acoustic horn.
 5. The apparatus of claim 1,wherein the resonance frequency of the acoustic device is in the rangeof 10 kHz to 150 kHz.
 6. The apparatus of claim 5 wherein the resonancefrequency of the acoustic device is in the range of 50 kHz to 100 kHz.7. The apparatus of claim 1, wherein the monitoring surface is locatedon or near one of the following devices switches, valves, sleeves,mandrels, downhole separators and sensors located in the wellbore. 8.The apparatus of claim 1 further comprising a deposit removal system forat least partially removing the deposition from the monitoring surface,the deposit removal system being in a control loop with said depositmonitor.
 9. The apparatus of claim 8, wherein the deposit removal systemincludes a deposition inhibiting or removing chemical agent.
 10. Theapparatus of claim 8, wherein the deposit removal system uses theacoustic device to exert a physical force onto the deposited material.11. The apparatus of claim 8, wherein the deposition removal system isnear a sensor having a surface exposed to the fluids and the depositionremoval system is for removing deposits from said exposed surface. 12.The apparatus of claim 11, wherein the sensor is selected from a groupcomprising optical sensors, electro-chemical sensors, or acousticsensors.
 13. The apparatus of claim 11, wherein the exposed sensorsurface is selected from a group comprising optical windows, membranes,or sensitive areas of acoustic sensors.
 14. The apparatus of claim 1,comprising an additional sensing system to analyze material deposited onthe monitoring surface.
 15. A monitoring apparatus located in ahydrocarbon wellbore comprising: a monitor for measuring characteristicsof fluids in the hydrocarbon wellbore, the monitor having a monitoringsurface that is directly exposed to fluids in the hydrocarbon wellbore;a deposit removal system including an acoustic device for exerting aphysical force on the monitoring surface to at least partially remove adeposition of material from the monitoring surface; and a power supplyfor supplying said acoustic device with electrical energy, wherein themonitor further uses the acoustic device, said acoustic device to beoperated in a resonance mode, wherein the monitor measures deposition ofthe material on the monitoring surface by measuring a change inresonance frequency of the acoustic device and wherein the acousticdevice operates in a longitudinal mode.
 16. The apparatus of claim 15,wherein the acoustic device further comprises a transducer, and afocussing element coupled to the transducer.
 17. The apparatus of claim15, wherein the resonance frequency of the acoustic device is in therange of 10 kHz to 150 kHz.
 18. The apparatus of claim 15, wherein thedeposit removal system includes a deposition inhibiting or removingchemical agent.
 19. A monitoring apparatus located in a hydrocarbonwellbore comprising: a monitor for measuring characteristics of fluidsin the hydrocarbon wellbore, the monitor having a monitoring surfacethat is directly exposed to fluids in the hydrocarbon wellbore; adeposit removal system including an acoustic device for exerting aphysical force on the monitoring surface to at least partially remove adeposition of material from the monitoring surface; and a power supplyfor supplying said acoustic device with electrical energy, wherein themonitor is selected from a group comprising optical sensors,electro-chemical sensors, nuclear sensors, separation membranes, oracoustic sensors separate from the force exerting acoustic device. 20.The apparatus of claim 19, wherein the monitor is a gamma ray densitymeasurement system.
 21. The apparatus of claim 20 wherein the monitoringsurface is a nuclear window.
 22. The apparatus of claim 19, wherein themonitor is an optical fluid analyzer.
 23. The apparatus of claim 22wherein the monitoring surface includes an optical window.
 24. Theapparatus of claim 19, wherein the monitor is used to measure activityof an ionic species contained in the wellbore fluid.
 25. The apparatusof claim 24 wherein the monitoring surface is a membrane of an ionselective electrode.
 26. The apparatus of claim 19, wherein themonitoring surface is a separation membrane.
 27. A deposit monitoringapparatus located in a hydrocarbon wellbore comprising: an acousticdevice for operating in a resonance mode including a monitoring surfacedirectly exposed to fluids in a hydrocarbon wellbore, wherein thedeposition of material on the monitoring surface is monitored bymeasuring a change in resonance frequency of the acoustic device, andwherein by measuring said change in resonance frequency of the acousticdevice a thickness of deposited material of 600 microns can bedistinguished from a thickness of deposited material of 1050 microns;and a power supply for supplying said acoustic device with electricalenergy, wherein the acoustic device operates in a longitudinal mode. 28.The apparatus of claim 27, wherein the acoustic device further comprisesa transducer, and an acoustic horn coupled to the transducer.
 29. Theapparatus of claim 27, wherein the resonance frequency of the acousticdevice is in the range of 10 kHz to 150 kHz.